This invention relates to rotor structures for motors, a motor using such a rotor and an electric power assisted steering system using such a motor.
Electric Power Assisted Steering (EPAS) systems are well known in the prior art. They typically comprise an electric motor having a stator and a rotor, an input shaft operatively connected to the rotor and adapted to rotate therewith, an output shaft associated with a steering column, and a gearbox adapted to transfer torque from the input shaft to the output shaft in response to a measure of torque in the output shaft produced by a torque sensor. The motor is typically operated to apply an increasing torque to the output shaft as the measured torque increases, thus applying an assistance torque which helps to steer the vehicle. Such a system is disclosed in PCT Patent Application publication number WO99/65758.
It is desirable to be able to determine the angular position of the rotor of the motor of such a system. Typically, this is achieved by having an encoder member such as a disc fixed by means of rivets or other such fastenings to the rotor. The encoder member carries a pattern encoding angular position, such as a magnetic or optical pattern. A sensor fixed relative to the motor can then sense the pattern and from that determine the position of the rotor.